Ultrashort laser pulses have many uses which range from information technology to material processing. To generate ultrashort laser pulses, fiber lasers, for example, are used in which the laser light is guided in an optical fiber in the resonator. A correspondingly doped optical fiber or a doped fiber section serves as a laser amplifier medium. Depending on the laser wavelength, different doping elements are possible, especially rare earths.
Mode-locked ultrashort pulse fiber lasers are known which are constructed in a figure-of-eight configuration. Such a laser is known for example from US 2008/0144676 A1. Mode-locking is active or passive. In the event of active mode-locking, and electro-optical or acousto-optical modulator is arranged in the resonator.
For mode-locked ultrashort pulse fiber lasers, fibers or fiber sections with normal and abnormal dispersion are frequently combined. In the context of the present description, “dispersion” stands for group delay dispersion, i.e., pulse propagation per spectral bandwidth (in an angular frequency). In the event of normal dispersion in a fiber with a fixed length, the group speed decreases with the frequency. In the event of abnormal dispersion, the group speed increases with the frequency.
By combining optical fibers with opposing dispersion, it is possible to compensate for dispersion arising in the fibers as the light pulses pass through. Depending on the respective spectral range, optical fiber modules with corresponding dispersion for dispersion compensation with simultaneous monomode operation are difficult to obtain and are correspondingly laborious to create and expensive. Furthermore, it is technically demanding to splice the optical fiber module to a conventional fiber.
US 2008/0144676 A1 proposes a design in which components are used that have normal and abnormal dispersion. In order to start the ultrashort pulse fiber laser, a modulator is integrated in one of the two rings of the figure-of-eight configuration. This is coupled to an external signal source which is operated at the frequency of the laser.
A design for an ultrashort pulse fiber laser is found in C. Aguergaray, R. Hawker, A. F. J. Runge, M. Erkintalo, and N. G. R. Broderick: “120 fs, 4.2 nJ pulses from an all-normal-dispersion, polarization-maintaining, fiber laser”, Appl. Phys. Lett. 103, 121111 (2013) with an optical bandpass filter integrated in its figure-of-eight configuration. This serves to generate self-consistent pulse propagation and support passive mode-locking. A modulator does not have to be used in this context since the laser is self-starting. A laser configuration consisting entirely of normally dispersive components and a bandpass filter is also termed a so-called ANDI design (all normal dispersion).
Another fiber laser is known from US 2008/0025348 A1. This comprises a main ring and a reinforcing ring which are constructed from fibers with different dispersion. The reinforcing ring is constructed from fibers with a positive dispersion, and the main ring is constructed, at least in sections, from fibers with a negative dispersion. To achieve complete dispersion compensation, a dispersion compensator constructed from fibers is also integrated in the main ring. This comprises two chirped fiber Bragg gratings with which differential dispersion compensation is achieved.